Some types of power amplifiers such as pulse width modulated (PWM) amplifiers include a network of switching elements for controlling the directional flow of output current into a load. By outputting currents that alternate in direction, PWM amplifiers drive direct current (DC) and stepper motors for motion control applications in robotics, servomechanisms, printing devices, etc.
To provide currents with alternating flow directions, some PWM amplifiers implement four switching elements that provide two output currents with different flow directions. This circuitry, known as an “H-Bridge”, may include various types of electronic components (e.g., relays, transistors, etc.) to provide the four switching elements.
To control H-Bridge operations, the PWM amplifier produces a pulse train that controls the functioning of the electronic switching components. For example, an external signal provided to a PWM amplifier may control the duty cycle of the pulse train. To initiate current flow in one direction, the duty cycle of the pulse train is increased to one pair of switching elements while the duty cycle of a complementary pair of switching elements is reduced.
Conventional PWM amplifiers implemented in monolithic integrated circuits (ICs) typically implement n-channel transistors and are typically unable to independently provide appropriate signal levels for controlling H-Bridge operations. To attain the appropriate signal levels, such PWM amplifiers use transistors implemented as source followers to “pull-up” signal levels. These pull-up transistors are typically coupled using relatively large capacity capacitors, known as bootstrap capacitors. Due to their large storage capacity, these bootstrap capacitors are typically located external to the IC. By implementing pull-up transistors and bootstrap capacitors, design complexity and production cost increases.